The cryptophycins, natural products isolated from blue-green algae, are under study. The cryptophycins are currently considered one of the most exciting new leads in cancer chemotherapy. They have been identified as extraordinarily potent antitumor agents with demonstrated excellent in vivo activity in subcutaneously transplanted solid tumors of mouse and human origin. Of special importance are the findings that they possess tumor-selective toxicity and greatly reduced susceptibility to P-glycoprotein-mediated multiple drug resistance in comparison to vinblastine, colchicine and taxol. The cryptophycins are antimitotic agents, interacting with tubulin in a vinca-alkaloid-like fashion. We plan to prepare cryptophycin fluorescent, photoaffinity and electrophilic labels to characterize the tubulin binding domain of cryptophycin. The fluorescent analogues will be used to define the solvent accessibility of different regions of the molecule when bound to tubulin. The photoaffinity and electrophilic analogues will be used to map the cryptophycin binding domain of tubulin. Since the different functional groups of cryptophycin are expected to interact with a variety of amino acid residues in the binding site, we plan to attach fluorescent and photoaffinity probes to different positions of the cryptophycin structure in an effort to map the different contact points between the cryptophycin molecule and the tubulin amino acids. We intend to prepare analogues carrying labels at C3, C6, C7, C10, C13, and at the C3'-phenyl ring of the molecule. The analogues will be investigated in a microtubule assembly assay and in cell cultures of cancer cells. Suitable candidates will be selected and used to study the ligand-protein interactions in fluorescent, photoaffinity and electrophilic labeling studies.